The present invention relates to a hydrolysis catalyst intended for use in a combustion engine, particularly as part of an exhaust system, and more particularly to an improved construction of the hydrolysis catalyst.
A known practice in combustion engines is for the exhaust gases to be treated by various post-treatment systems with the object of reducing the proportion of harmful constituents in the exhaust gases. A known way of reducing the proportion of nitrogen oxides is to use an SCR catalyst in which ammonia being mixed with the exhaust gases makes it possible to reduce the nitrogen oxides to nitrogen gas and water. To avoid injecting pure ammonia into the exhaust gases, what is used instead is a more easily handled substance, usually a uric substance, such as an aqueous solution of urea, which when injected into the exhaust gases is converted first to ammonia which thereafter reacts with the nitrogen oxides in the SCR catalyst. A known practice with a view to facilitating the conversion of urea to ammonia is to use a hydrolysis catalyst in which urea is converted to ammonia and carbon dioxide before these are led into a downstream SCR catalyst. An example of such a solution is described in EP 0907010.
A problem which arises in the injection of an aqueous solution of urea is that it readily crystallises if it encounters cold surfaces. With a view to preventing this, there are various solutions intended to ensure that the injected urea is at a sufficiently high temperature when it is introduced into the SCR catalyst and/or prevent contact with cold surfaces in the exhaust line. When the exhaust temperature rises, crystallised urea will certainly be dissolved but may, before that happens, cause blockages in the exhaust line or other serious operational malfunctions.
Even if it is not absolutely necessary to use a hydrolysis catalyst before the SCR catalyst, it affords advantages in ensuring better conversion of urea to ammonia and supporting the intended function of the SCR catalyst. However, a problem which is particularly evident during low-load operation is that urea crystallises and obstructs the ducts in the hydrolysis catalyst.
In the state of the art there are solutions which describe how a bypass line is provided to lead exhaust gases past a hydrolysis catalyst. For example, EP 1052009 refers to a solution with a bypass line so arranged that only a partial flow of exhaust gases passes the hydrolysis catalyst. The result is that the urea in the exhaust gases which pass through the hydrolysis catalyst reacts better and more completely converts the urea to ammonia. The amount of exhaust gases led through the bypass line is controlled under the influence of a damper device on the basis of detected engine parameters such as exhaust flow, exhaust velocity and/or exhaust pressure. The fact that this device comprises a number of additional components makes this a solution which is both bulky and expensive.
A requirement for satisfactory functioning of a hydrolysis catalyst is that the uric substance is completely allowed to be converted to ammonia and carbon dioxide. Even if it is possible to lengthen the dwell time in the catalyst in the manner described in the aforesaid EP 1052009, its active surface needs to be sufficiently large. In practice this is achieved by providing as many cell walls as possible within a limited region, which leads to the ducts bounded by these walls having relatively small cross-sectional areas. This unfortunately leads to the possibility that the pressure drop across a hydrolysis catalyst may be considerable.
A further requirement for a hydrolysis catalyst to function in an intended manner is that it reaches an intended operating temperature. This entails reduction of its functioning during warm-up and during low-load running, which may be catered for by, for example, external warming by electrical means. This involves the disadvantage that the necessary additional components occupy space and that the costs increase.